How is "knowing nothing whatever about him" a "subtle tribute" paid to Gatsby by his guests?

Most of the guests at Gatsby's opulent parties don't know anything about him. And in a way, that's a good thing, on account of their host's somewhat shady past. If there's ever a police raid on one of their little soirees, then they can disclaim all knowledge of who's been serving them all that hooch. Not knowing anything about their host also allows Gatsby's guests to drink as much illegal booze—remember, the story's set during Prohibition—without inquiring too closely into its origins.
Ignorance of Gatsby's true identity makes his parties a lot more fun. That way his guests can enjoy indulging in idle gossip about him, making up all kinds of tall stories about the legend that is Jay Gatsby. Nick is being slightly sarcastic in describing this as a "subtle tribute." It's so subtle, it's not really a tribute at all. Gatsby's freeloading guests really don't want to know anything about him at all; they simply want to show up to his parties, sponge off his lavish hospitality and continue to tell increasingly lurid stories about this strange, mysterious individual.